Zeolites may be modified by certain traditional approaches. Some of these traditional approaches are made to change and control the silicon to aluminum ratio in the zeolite, replace the exchange ion in the zeolite or passivate the surface of the zeolite crystals. Some of the modifications have secondary effects such as change in the acidity of the aluminum centers, in hydrophobicity of the zeolite crystals and in interaction with various other molecules. Such traditional approaches include acid dealumination treatment, hydrogen fluoride treatment, silicon tetrachloride surface passivation, steaming and others.
With respect to dealumination processes, each process results in a particular change to the zeolite structure. For example, steaming, an established method for de-alumination of certain zeolites, results in a gradient of the aluminum concentration across the zeolite crystal, with aluminum highest on the outside of the crystal and lowest in the core. In contrast, dealumination performed with aqueous acid is significantly more uniform. Steaming also leaves a significant amount of extra framework aluminum oxide, which is completely absent in the aqueous acid dealumination.
Zeolite catalysts have been used convert mixtures of methyl bromide and dibromomethane to hydrocarbon products. Often, when methyl bromide/dibromomethane are reacted over zeolite catalysts or zeolite catalysts that have been further processed by SiCl4 surface treatment, traditional dealumination methods such as steaming and steaming followed by acid treatment, acid dealumination by various acids, trimethoxyphosphine treatment, ion exchange and some combinations of the mentioned modification techniques, dibromomethane quickly poisoned the catalyst. This poisoning resulted in a rapid decrease in conversion and coke formed in quantities exceeding the content of dibromomethane in the feed. Consequently, traditionally, it may be necessary to remove dibromomethane from the methyl bromide/dibromomethane stream prior to reaction with the zeolite catalyst.